The present invention is directed to an improved flexible cutting line for use in rotary trimmers and the method and apparatus employed in the manufacture of such line. Flexible line rotary trimmers are used for cutting vegetation, such as grass and weeds, particularly along walks, fences and flowerbeds and around trees. These devices comprise a motor-driven rotary head which carries one or more lengths of monofilament line mounted on a spool within a housing. Extended end portions of each line project from the spool through guides in the side of the housing. As the head rotates at high speed, the end portions of the line are caused to project outwardly from the housing by the centrifugal forces acting thereon and function as cutting blades. The majority of trimmer heads presently in use employ two separate monofilament nylon lines which are both mounted on a common spool and project from the spool and housing through diametrically opposed guides in the trimmer housing.
The speeds at which these rotary trimmers rotate is generally within the range of 5,000 to 10,000 rpm. At these speeds, the projecting nylon lines generate considerable noise which most people find quite objectionable. In trimmers having electric motors, this noise is considerably louder than that generated by the motor. As a result, efforts have been ongoing to develop a suitable cutting line for such trimmers that generate less noise during use. In gas-powered trimmers, the drive engines are quite loud. However, the high pitch whine given off by the nylon slicing through the air and brush appears to travel further than the lower frequency engine noise and is very noticeable to people not in the immediate proximity of the rotating trimmer head. Thus, a noise attenuating cutting line would also be quite useful in gas-powered trimmers. As legislative pressure continues to force manufacturers to develop quieter gas-powered engines for these trimmers, the need for a noise attenuating cutting line will only increase.
While efforts to develop suitable cutting lines for rotary trimmers that produce less noise during use have met with some success, the noise attenuating characteristics of such lines still leaves room for improvement. In addition, the configurations of such lines generally require secondary processing steps, such as notching or cutting grooves of varying numbers in the line. Tightly twisting a formed filament of square cross-section has also been employed to attenuate noise. However, as a result of such modifications, the surface quality and durability of the line is adversely affected and the cost of manufacture is significantly increased. It therefore would be highly desirable to produce a flexible cutting line for rotary trimmers that not only generates less noise than those lines currently in use but also is quite durable and capable of being manufactured without the need for costly secondary processing. It would also be desirable if the efficiency of the line slicing through the air could be sufficiently increased such that not only was the noise of the rotating line significantly attenuated but also the vibration in the rapidly rotating head and the drag on the drive motor were reduced as well. Such an improvement would reduce the strain on the operator and, in the case of gas-powered trimmers, reduce fuel consumption and emissions. When used with heads powered by electric motors, such line would reduce the amperage drawn by the motor and, with battery-powered trimmers, it would extend the life of the batteries. The flexible cutting line of the present invention obtains these results.